The present invention relates to a zoom lens system for a projector.
As one example of zoom lens systems for projecting an image onto a screen from a light modulating device, such as a DMD (“digital mirror device” or “digital micro device”), equipped with a plurality of elements that form an image by changing the direction of reflection of light, a zoom lens system that includes a first lens system with negative refractive power and a second lens system with positive refractive power in that order from the screen side is known.
The zoom lens disclosed by Japanese Laid-Open Patent Publication No. 2002-148515 consists of a total of ten lenses with four lenses in the first lens group and six lenses in the second lens group, and has a zoom ratio of 1.2.
The zoom lens disclosed by Japanese Laid-Open Patent Publication No. 2004-240309 consists of a total of twelve lenses with five lenses in the first lens group and seven lenses in the second lens group, and also has a zoom ratio of 1.2.
A zoom lens system composed of two lens groups with respectively negative and positive refractive powers has a simple construction and is compact, thereby making it suited to a low-cost projector. Accordingly, the use of this type of zoom lens system in a portable projector, a consumer projector, or the like is being investigated. To project enlarged images that are bright and sharp, a zoom lens system for a projector needs to be bright, i.e., to have an F number of around 2.4 to 3, and to favorably correct aberration. In addition, a zoom lens system of a reflection-type projector that uses a DMD needs to have a sufficiently long back focus to provide sufficient space for light to be irradiated onto the DMD to illuminate the DMD.
Although there are also demands for higher zoom ratios, it is not easy to increase the zoom ratio in addition to satisfying the demands described above for a zoom lens system used in a projector. For example, for the zoom lens systems disclosed in Patent Documents 1 and 2, sufficient aberration correcting performance is achieved by setting the refractive powers of the first lens group and the second lens group substantially equal. Since it is difficult to obtain sufficient peripheral light when the length of the lens is increased relative to the back focal length at the wide angle end, the zoom ratio is suppressed to around 1.2.